Do not clean old coins you suspect might be of value and you intend to sell. Even rubbing the dirt off of a coin find with your fingers can scratch and lower the value of a coin. Soak in water to remove dirt, never rub it with anything. A rare coin is more valuable to coin buyers dirty than one that has been cleaned by an amatuer. Let them clean it as they are pros and you will get more $$$ even with the dirt on.
This can apply to tarnished coins to that have not been dug up, or in collections. I know its tough but leave them the way they are! The grading of rare coins is a tough, cut throat biz and you can really screw up a valuable coin by messing with it.
